1. Field of the Invention
The present invention relates to a method and apparatus for controlling a supercharged engine equipped with a supercharger operable to supercharge fresh air to be introduced into a combustion chamber of the engine.
2. Description of the Background Art
Heretofore, in a spark-ignition gasoline engine having a spark plug, a method has been implemented which is designed to perform a compression self-ignition combustion based on self-ignition of an air-fuel mixture, in a partial engine load operating region, and to perform a spark-ignition (SI) combustion based on forced ignition of an air-fuel mixture by a spark plug, in another operating region on a higher engine load side than the partial engine load operating region, as disclosed in JP 2007-292060A.
As for the method designed to selectively use the compression self-ignition combustion and the spark-ignition (SI) combustion depending on engine load in the above manner, various techniques have heretofore been proposed. Specifically, the compression self-ignition combustion is a combustion mode where an air-fuel mixture self-ignites simultaneously at many positions in a combustion chamber, and considered to have a potential to provide higher efficiency than the commonly-used spark-ignition combustion. However, the compression self-ignition combustion has a problem with combustion controllability (i.e., preignition or knocking is more likely to occur) when the engine load is high, so that it is necessary to perform the spark-ignition combustion with high controllability, in a high engine load range. For this purpose, the two combustion modes, i.e., the compression self-ignition combustion and the spark-ignition combustion, are selectively used depending on engine load. This allows the compression self-ignition combustion to be adequately performed, but partially, so that engine thermal efficiency is enhanced in an engine operating region subject to the compression self-ignition combustion, which provides an advantage of being able to improve fuel economy performance.
Actually, the compression self-ignition combustion is apt to become unstable not only in the high engine load range but also when the engine load is extremely low (e.g., during idling). Specifically, when the engine load is extremely low, an internal temperature of a cylinder of the engine (in-cylinder temperature) is lowered, and thereby misfire is more likely to occur. In view of this problem, JP 2007-85241A proposes a technique of, in an extremely-low engine load range of an engine, forcedly heating intake air (fresh air) by means of heat exchange with exhaust gas, and introducing the heated air into a cylinder. Based on introducing fresh air into the cylinder after heating it in the above manner, the in-cylinder temperature is raised to promote self-ignition, which provides an advantage of being able to prevent misfire.
Meanwhile, it is known that, as means for enhancing the engine thermal efficiency, a technique may be employed which is designed to burn an air-fuel mixture under an air-fuel ratio leaner than a stoichiometric air-fuel ratio. Specifically, when air is introduced into the cylinder in an excess amount with respect to a fuel (gasoline) supply amount to perform combustion under a resulting lean air-fuel ratio, a combustion temperature can be lowered as compared with combustion under the stoichiometric air-fuel ratio, so that an exhaust loss and a cooling loss of the engine are reduced, which makes it possible to further enhance the engine thermal efficiency.
Therefore, if the compression self-ignition combustion can be performed under the condition that the air-fuel ratio is set to a lean value, it is expected to make it possible to more effectively enhance the engine thermal efficiency so as to improve the fuel economy performance.
However, in cases where the compression self-ignition combustion is performed under a lean air-fuel ratio, there is concern that misfire is more likely to occur in a low engine load range of the engine. As measures against the misfire, it is contemplated to employ the technique of heating fresh air as disclosed in the JP 2007-85241A. However, in a situation where the engine load is low under a lean air-fuel ratio, if it is attempted to produce self-ignition forcibly by means of heating of fresh air, it is assumed that there is a need for considerably heating fresh air, which gives rise to concerns, such as problem in terms of practicability, and reduction in engine power output due to lowering of air density.